A solar cell geometry referred to as an interdigitated back contact (IBC) cell comprises a semiconductor wafer and alternating lines (interdigitated stripes) of metal coinciding with regions with p-type and n-type doping, respectively. This cell geometry has the advantage of eliminating shading losses altogether by putting both contacts on the rear side of the wafer that is not illuminated. Further, interconnection in a module is facilitated with both contacts on the rear surface of the wafer.
Presently, silicon solar cells with the highest efficiency are those based on the IBC structure, with cells exhibiting efficiencies as high as 26.4%. While the processing of these high efficiency IBC solar cells was not discussed in any detail, the manufacturing costs are likely to be relatively high since the known processing techniques that could be applied in each appear to be somewhat complicated with various masking and vacuum processing steps required.
Many solar cell structures, including the IBC structure, rely upon thin fingers of metal to collect current. In the case of the IBC cell, these thin fingers are interdigitated on the back of the cell, and often resulting from a single metal deposition that has been patterned to reveal the finger structure. The area of each contact is less than ½ of the total cell area because they share the same layer and there is need for an insulation region (e.g. isolation gap) between the fingers. Combining this with the fact that current must travel along the narrow fingers, very high conductivity layers are required for the fingers. The solar industry resorts almost exclusively to silver, an expensive metal, to address the need for high conductivity, thus low resistance. Additionally, larger cell sizes exacerbate issues associated with resistance in the metal contacts of the solar cell.
There have been attempts to provide improved metallization for solar cells. A multilayer metallization applies each contact metal (base, emitter) as a contiguous sheet of metal over the entire cell with vias in the bottom contact metal sheet allowing the top sheet to make localized base contacts, such vias made with a photolithographic process. However the photolithographic process is expensive and not suited to solar cell manufacturing. Another method applies relatively thick metals in cost effective ways, but does not address approaches needed for make useful back contact cells. Therefore, there is firstly a need for effective and inexpensive methods to provide metallization for back contact cells.
Furthermore, in order to allow the basic operation of a solar cell and extract electric current, regions of the solar cell may behave so that they favor the existence of one charge carrier (holes or electrons) over the other. Conventionally, these regions may be formed by doping the silicon with dopant atoms that convert the silicon to a state where a surplus of electrons (n-type) or holes (p-type) is favored. Doping, however, usually requires high temperature processes since the dopant needs to be incorporated in the silicon above its melting point or at a sufficiently high temperature that it can diffuse into solid silicon.
An alternative approach is to contact the silicon with materials or systems that energetically favor the existence or transport of a particular charge carrier. These materials or systems may also need to sufficiently passivate the surface of the silicon to minimize defects at or near the surface and avoid fermi level pinning, reducing the likelihood of recombination or energetic barriers at the contact. Such materials or systems may be referred to as heterocontacts, passivating contacts, or carrier selective contacts. High temperature doping, whether achieved with thermal diffusion, thermal annealing, or laser doping, introduces defects in the solar cell that limit performance. Carrier selective contacts may be applied at low temperature and cause minimum damage to the substrate, and thus are capable of higher performance.
In order to produce very high performance solar cells at low cost, there is a need to provide multilevel metallization systems in conjunction with carrier selective contacts. There is also a need to provide carrier selective contacts in a low cost process. In U.S. patent application Ser. No. 15/068,900 filed on Mar. 14, 2016 (U.S. Pub. No. 2017/0062633), which is fully incorporated by reference herein, multilayer foil metallization with laser processed back contacts were discussed. Further improvements to systems and methods for forming rear emitters and/or rear base contacts using carrier selective contacts with multilayer foil metallization are discussed herein.